1. Field of the Invention
This invention generally relates to the field of gas ionization and more particularly to apparatus for ionizing gas in a local area.
2. Description of Related Art
Certain properties of ionized gas make it attractive to different industrial applications, particularly, those involving the control of an electrostatic charge on a body. In such applications, a gas ionizing apparatus produces an ionization and electrical field that causes the air molecules in the vicinity of the apparatus to break into positive and negative electrons. As opposite charges attract, any charged material passing near the apparatus will attract ions until the charge is neutralized.
In accordance with the prior art, gas ionizing apparatus typically includes an insulator that supports a series of spaced, discrete pointed electrodes proximate a ground reference frame that is isolated from the electrodes. When a voltage of about 3 KV or more is applied across the electrodes and frame, a discharge occurs that ionizes gas in the vicinity of the electrode.
The spacing of the electrodes can produce discrete regions of ionization. More specifically, as a pointed electrode tip becomes dull through use or is coated with dust or other materials from the environment, the ionization rate from that electrode can reduce or even stop an obvious expedient for improving uniformity by merging the discrete regions might be realized by reducing the electrode spacing. However, there is minimum spacing beyond which further decreases in the spacing reduce the ionization from each electrode. In some situations ionization from one electrode may even stop. Moreover, the problems of dulling the points and accumulations on the points continue to exist. Consequently, these prior art devices do not readily respond or adjust to these situations.
Charged or grounded conductive brushes have also been used to control charges on a body. For example, the following United States Letters Patent discloses apparatus for ionizing gas to neutralize the charge on a moving web:
U.S. Pat. No. 4,307,432 (1981) Nishikawa
In accordance with U.S. Pat. No. 4,307,432, a charge neutralizing apparatus includes a brush of thin conductive wires. Free ends of the wires that are brought into proximity with, but spaced from, a charged web. The charge on the web produces an electric field at the free end of a wire that allows gas at the free ends of the brush to ionize and neutralize any charge on the material. The range of spacing between the brush ends and the material is limited to about 0.5 mm to 2 mm. Moreover, the rate of ionization is dependent upon the charge on the moving web. Consequently, it is difficult to control the rate of ionization. Moreover, it is difficult to position the brushes at any significant distance from the material being treated. Finally, the structure does not constitute an independent, self-contained gas ionizer.
The following patents disclose apparatus for controlling the charge on a material generally in response to an electric field:
______________________________________ 4,336,565 (1982) Murray et al 4,363,070 (1982) Kisler 4,383,752 (1983) Kisler 4,517,143 (1985) Kisler 4,810,432 (1989) Kisler 4,825,334 (1989) Kisler ______________________________________
U.S. Pat. No. 4,336,565 discloses a process for imposing an electrical charge on an electrically insulating surface of a moving web. A brush, comprising extremely soft and flexible fiber filaments such as carbon filaments, contacts one surface of the web and transfers a charge to that surface. This apparatus produces an electric field between the brush and a ground plane disposed on the other side of the web.
In U.S. Pat. No. 4,363,070 a brush-like device has an array of conductive filaments for neutralizing electrostatic charges on a moving web. In this apparatus a web of charge-retaining material passes a grounding brush and a second brush that subjects the material to an alternating electrical field. The alternating field is generated between the second brush and a grounded roller that are oppositely disposed with respect to the web. An electrostatic charge sensor and related control circuitry maintain the web at a substantially uncharged condition.
U.S. Pat. No. 4,383,752 discloses a power supply for producing an electric field between a brush electrode and a rotating roller disposed oppositely with respect to a web. This patent also discloses a current limiting resistor in series with the conductive brush.
U.S. Pat. No. 4,517,143 discloses a method and apparatus for establishing a uniform charge level on a randomly charged web. The apparatus generates first and second uniform electrostatic fields of different magnitudes and of opposite polarities between grounded rollers and spaced brush electrodes. As a web moves between each of the brush electrodes and its respective roller, a charge accumulates on the material. U.S. Pat. No. 4,810,432 describes a similar structure. This patent further discloses apparatus for mechanically vibrating the brush electrodes to assure a uniform charge distribution.
In accordance with U.S. Pat. No. 4,825,334 a power supply energizes a conductive brush with respect to a grounded roller that transports a web material between the roller and brush. The apparatus prevents the production of corona at the tips of the individual brush bristles by limiting current to the microamp range.
Other patents that utilize bristle brushes primarily for neutralizing charge are disclosed in the following patents:
______________________________________ 4,579,441 (1986) Hart et al 4,771,369 (1988) Ayash 4,994,861 (1991) Brandon et al ______________________________________
U.S. Pat. No. 4,579,441 discloses an electrostatographic apparatus that includes an imaging surface with an electrostatic device for transferring toner particles. A plurality of conductive fibers extend across the rear side of a receiving sheet and are spaced from the sheet. A low voltage direct current .potential applied to those fibers has a polarity opposite the polarity on a receiving sheet to neutralize the charge.
U.S. Pat. No. 4,771,360 depicts a brush for grounding a component that includes a support member having a multiplicity of fibers mounted thereon. The grounding brush contacts various rolls and shafts of a printing machine thereby to dissipate any electrostatic charges.
The apparatus in U.S. Pat. No. 4,994,861 performs a similar charge neutralizing function. A grounded wire located in proximity to a sheet neutralizes any accumulated charge on the sheet. This system includes a grounded conductive brush mounted on a sheet deflector so that one end of the brush contacts sheets as they traverse a first sheet feed path.
Apparatus relying on electrostatic fields has certain common characteristics. Each generates an electric field between a brush conductor and a spaced electrode for acting on an intermediate web. Consequently each requires a separate grounding electrode that must be supported on a machine independently of the conducting brush. In these structures the spacing between the ends of the brush bristles and the web and the spacing between the brush bristles and the complementary machine electrode are critical to the strength and uniformity of the electrostatic field and the efficiency with which the apparatus charges or neutralizes the web. Certain of the devices, particularly the grounding brushes, maintain physical contact with the material being modified thereby providing friction and potential surface marring. None of the references discloses a device that constitutes a self-contained, independent, non-contacting apparatus for controlling the charge on a web.